High speed low power magnetic devices based on current induced spin-momentum transfer

ABSTRACT

The present invention generally relates to the field of magnetic devices for memory cells that can serve as non-volatile memory. More specifically, the present invention describes a high speed and low power method by which a spin polarized electrical current can be used to control and switch the magnetization direction of a magnetic region in such a device. The magnetic device comprises a pinned magnetic layer with a fixed magnetization direction, a free magnetic layer with a free magnetization direction, and a read-out magnetic layer with a fixed magnetization direction. The pinned magnetic layer and the free magnetic layer are separated by a non-magnetic layer, and the free magnetic layer and the read-out magnetic layer are separated by another non-magnetic layer. The magnetization directions of the pinned and free layers generally do not point along the same axis. The non-magnetic layers minimize the magnetic interaction between the magnetic layers. A current is applied to the device to induce a torque that alters the magnetic state of the device so that it can act as a magnetic memory for writing information. The resistance, which depends on the magnetic state of the device, is measured to thereby read out the information stored in the device.

This patent application is a continuation of application Ser. No.10/643,762 filed Aug. 19, 2003, now U.S. Pat. No. 6,980,469 allowed Sep.12, 2005.

This invention was made with government support under Contract NumberNSF-FRG-DMS-0201439 entitled “Analysis and Simulation of MagneticDevices” awarded by the National Science Foundation, and Contract NumberONR N0014-02-1-0995 entitled “Gate Controlled Ferromagnetism inSemiconductor Nanostructures” awarded by the Office of Naval Research ofthe Department of Defense. The government has certain rights in theinvention.

FIELD OF THE INVENTION

The present invention generally relates to magnetic devices used inmemory and information processing applications, such as giantmagnetoresistance (GMR) devices. More specifically, the presentinvention describes a high speed and low power method by which a spinpolarized electrical current can be used to control and switch thedirection of magnetization of a magnetic region in such a device.

BACKGROUND OF THE INVENTION

Magnetic devices that use a flow of spin-polarized electrons are ofinterest for magnetic memory and information processing applications.Such a device generally includes at least two ferromagnetic electrodesthat are separated by a non-magnetic material, such as a metal orinsulator. The thicknesses of the electrodes are typically in the rangeof 1 nm to 50 nm. If the non-magnetic material is a metal, then thistype of device is known as a giant magnetoresistance or spin-valvedevice. The resistance of the device depends on the relativemagnetization orientation of the magnetic electrodes, such as whetherthey are oriented parallel or anti-parallel (i.e., the magnetizationslie on parallel lines but point in opposite directions). One electrodetypically has its magnetization pinned, i.e., it has a higher coercivitythan the other electrode and requires larger magnetic fields orspin-polarized currents to change the orientation of its magnetization.The second layer is known as the free electrode and its magnetizationdirection can be changed relative to the former. Information can bestored in the orientation of this second layer. For example, “1,” or “0”can be represented by anti-parallel alignment of the layers and “0” or“1” by parallel alignment. The device resistance will be different forthese two states and thus the device resistance can be used todistinguish “1” from “0.” An important feature of such a device is thatit is a non-volatile memory, since the device maintains the informationeven when the power is off, like a magnetic hard drive. The magnetelectrodes can be sub-micron in lateral size and the magnetizationdirection can still be stable with respect to thermal fluctuations.

In conventional magnetic random access memory (MRAM) designs, magneticfields are used to switch the magnetization direction of the freeelectrode. These magnetic fields are produced using current carryingwires near the magnetic electrodes. The wires must be small incross-section because memory devices consist of dense arrays of MRAMcells. As the magnetic fields from the wires generate long-rangemagnetic fields (magnetic fields decay only as the inverse of thedistance from the center of the wire) there will be cross-talk betweenelements of the arrays, and one device will experience the magneticfields from the other devices. This cross-talk will limit the density ofthe memory and/or cause errors in memory operations. Further, themagnetic fields generated by such wires are limited to about 0.1 Teslaat the position of the electrodes, which leads to slow device operation.Importantly, conventional memory designs also use stochastic (random)processes or fluctuating fields to initiate the switching events, whichis inherently slow and unreliable (see, for example, R. H. Koch et al.,Phys. Rev. Lett. 84, 5419 (2000)).

In U.S. Pat. No. 5,695,864 and several other publications (e.g., J.Slonckewski, Journal of Magnetism and Magnetic Materials 159, L1(1996)), John Slonckewski described a mechanism by which aspin-polarized current can be used to directly change the magneticorientation of a magnetic electrode. In the proposed mechanism, the spinangular momentum of the flowing electrons interacts directly with thebackground magnetization of a magnetic region. The moving electronstransfer a portion of their spin-angular momentum to the backgroundmagnetization and produce a torque on the magnetization in this region.This torque can alter the direction of magnetization of this region andswitch its magnetization direction. Further, this interaction is local,since it only acts on regions through which the current flows. However,the proposed mechanism was purely theoretical.

Slonckewski's patent describes MRAM devices that use spin-momentumtransfer for magnetic switching. However, the proposed devices are slowand rely on fluctuating magnetic fields and stochastic processes toinitiate magnetization switching. Further, large current densities areneeded to switch the devices. In describing the preferred embodiment ofhis “latch or logic gate,” Slonckewski states “ . . . the preferred axesof the 3 magnets F1, F2, and F3 are all “vertical” (i.e., in the samedirection or orientation) as discussed above. Other orientations canserve as long as they are parallel to the same axis.” As we describebelow, our device makes use of layer magnetizations that are notparallel to the same axis, to great advantage in speed, reliability, andpower consumption.

U.S. Pat. No. 6,256,223 to Jonathan Sun also describes devices that usecurrent-induced magnetic fields and demonstrates in experiment theoperation of such devices. However, the devices proposed wereunreliable, as there was little consistency with regard to devicecharacteristics. Further, the estimated time scale for magneticswitching was 50 nsec for operation at large current densities.

Devices are needed that exhibit high speed and reliable operation underthe action of a spin-polarized current. This includes devices thatoperate with lower power and have lower threshold currents for switchingthe magnetization orientation.

SUMMARY OF THE INVENTION

In view of the limitations associated with conventional designs ofdevices that use spin-momentum transfer, an object of the presentinvention is to provide a structure that is optimal for a magneticmemory or magnetic information processing device.

It is another object of the present invention to produce a magneticdevice that has advantages in terms of speed of operation.

It is a further object of the present invention to produce a magneticdevice that has advantages in terms of reliability.

It is a further object of the present invention to produce a magneticdevice that requires lower power to operate.

These and additional objects of the invention are accomplished by adevice that employs magnetic layers in which the layer magnetizationdirections do not lie along the same axis. For instance in oneembodiment, two magnetic regions have magnetizations that areorthogonal.

The invention is a magnetic device comprised of ferromagnetic andnon-magnetic layers through which current can flow. The magnetic deviceis comprised of a ferromagnetic layer with a fixed magnetizationdirection and another ferromagnetic layer separated from the first by anon-magnetic region that has a magnetization that is free to rotate inresponse to applied currents. A third ferromagnetic layer, again,separated from the others by a non-magnetic layer, has a fixedmagnetization direction and can be employed to readout the magnetizationdirection of the free ferromagnetic layer. The magnetization directionsof the ferromagnetic layers are not all along the same axis. In one ofthe preferred embodiments, the first fixed ferromagnetic layer'smagnetization direction is perpendicular to the plane of the layer,while the free ferromagnetic layer's magnetization is in the plane ofthe layer. As described above, a current flow between the layerstransfers spin-angular momentum from the fixed magnetization layer tothe free magnetization layer and produces a torque on the magnetizationof the free layer. The torque is proportional to the vector tripleproduct of the magnetization direction of the fixed and free layer, witha factor of proportionality that depends on the current and the spinpolarization of the current. A large torque is produced when themagnetization directions of the fixed and free layers are orthogonal.

This large torque acting on the magnetization direction of the freemagnetic layer causes the magnetization of the free magnetic layer torotate out of the plane of the layer. Since the thickness of the freemagnetic layer is less than the width and length dimensions, therotation of the magnetization of the free magnetic layer out of theplane of the layer generates a large magnetic field, a ‘demagnetizing’field, which is perpendicular to the plane of the layer.

This demagnetizing field forces the magnetization vector of the freemagnetic layer to precess, i.e., for the magnetization direction torotate around the direction of the demagnetization magnetic field. Thedemagnetizing field also determines the rate of precession. A largedemagnetizing field results in a high precession rate, which is anoptimal condition for fast magnetic switching. An advantage of thismagnetic device is that random fluctuating forces or fields are notnecessary to initiate or control the magnetic response of the layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will be morereadily apparent from the following detailed description and drawings ofthe illustrative embodiments of the invention wherein like referencenumbers refer to similar elements throughout the views and in which:

FIG. 1 is an illustration of a magnetic device according to the presentinvention;

FIGS. 2A–2E are illustrations of the free magnetic layer showing themagnetization vector and the demagnetizing field of the electronicdevice of FIG. 1 during the application of pulses of current asillustrated in FIG. 3A;

FIG. 3A is an illustration of a current waveform that may be applied tothe magnetic device;

FIG. 3B is an illustration of an alternate current waveform that may beapplied to the magnetic device;

FIG. 4 is an illustration of a memory cell according to one embodimentof the present invention;

FIGS. 5A–5E are illustrations of the free magnetic layer showing themagnetization vector and the demagnetizing field of the memory cell ofFIG. 4;

FIG. 6A is an illustration of a current waveform that may be applied tothe memory cell of FIG. 4 during a write operation;

FIG. 6B is an illustration of a resistance measured from the memory cellduring a read-out operation before and after the current pulse shown inFIG. 6A is applied;

FIG. 7 is an illustration of the free magnetic layer of a 4-state memorycell;

FIG. 8 is an illustration of an example of a current waveform applied tothe magnetic device;

FIG. 9 is an illustration of the magnetization components of the freemagnetic layer during and after the application of the current pulseshown in FIG. 8; and

FIG. 10 is an illustration of a memory cell according to one embodimentof the present invention in which during writing operations no netcurrent passes through the free magnetic layer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Structure of a Basic Magnetic Device

To illustrate the basic concept, FIG. 1 shows a multilayered,pillar-shaped magnetic device comprising a pinned magnetic layer FM1with a fixed magnetization direction and a free magnetic layer FM2 witha free magnetization direction. {right arrow over (m)}₁ is themagnetization vector of the pinned magnetic layer FM1, and {right arrowover (m)}₂ is the magnetization vector of the free magnetic layer FM2.The pinned magnetic layer FM1 acts as a source of spin angular momentum.

The pinned magnetic layer FM1 and the free magnetic layer FM2 areseparated by a first non-magnetic layer N1 that spatially separates thetwo layers FM1 and FM2 such that their mutual magnetic interaction isminimized. The pillar-shaped magnetic device is typically sized innanometers, e.g., it may be less than approximately 200 nm laterally.

The free magnetic layer FM2 is essentially a magnetic thin film elementimbedded in a pillar-shaped magnetic device with two additionallayers—the pinned magnetic layer FM1 and the non-magnetic layer N1. Thelayer thicknesses are typically approximately 1 nm to 50 nm.

These pillar-shaped magnetic devices can be fabricated in a stackedsequence of layers by many different means, including sputtering,thermal and electron-beam evaporation through a sub-micron stencil mask.These magnetic devices can also be fabricated in a stack sequence usingsputtering, thermal and electron-beam evaporation to form a multilayeredfilm followed by a subtractive nanofabrication process that removesmaterials to leave the pillar-shaped magnetic device on a substratesurface, such as that of a silicon of other semiconducting or insulatingwafer.

Materials for the ferromagnetic layers include (but are not limited to)Fe, Co, Ni, and alloys of these elements, such as Ni_(1-x)Fe_(x); alloysof these ferromagnetic metals with non-magnetic metals, such as Cu, Pd,Pt, NiMnSb, at compositions in which the materials are ferromagneticallyordered at room temperature; conducting materials; and conductingmagnetic oxides such as CrO₂ and Fe₃O₄. For the nonmagnetic layers,materials include (but are not limited to) Cu, Cr, Au, Ag, and Al. Themain requirement for the non-magnetic layer is the absence of scatteringof the electron spin-direction on a short length scale, which is lessthan about the layer thickness.

An electric current source is connected to the pinned magnetic layer FM1and the free magnetic layer FM2 so that an electric current I cantraverse the pillar device.

Method of Magnetic Switching

An electric current I is applied to the pillar-shaped magnetic device sothat the current I flows through the various layers of the device, fromthe pinned magnetic layer FM1 to the first non-magnetic layer N1 to thefree magnetic layer FM2. The applied current I results in a transfer ofangular momentum from the pinned magnetic layer FM1 to the free magneticlayer FM2. As stated above, a transfer of angular momentum from onemagnetic region to another can produce a torque.

FIGS. 2A–2E show steps in the method of magnetic switching using themagnetic device shown in FIG. 1 and for convenience, FIGS. 2A–2E onlyshow the free magnetic layer FM2 and the magnetization vector {rightarrow over (m)}₂ of the free magnetic layer FM2. FIG. 2A shows theinitial state of the free magnetic layer FM2 before the current I isapplied.

As shown in FIGS. 2B–2D, applying a current I, which can be of a form asshown in FIGS. 3A and 3B, results in the transfer of angular momentumfrom the pinned magnetic layer FM1 to the free magnetic layer FM2. Thistransfer of angular momentum from the pinned magnetic layer FM1 to thefree magnetic layer FM2 produces a torque {right arrow over (τ)}_(S) onthe magnetic moment of the free magnetic layer FM2.

The torque {right arrow over (τ)}_(S) per unit magnetization of the freelayer is proportional to the vector triple product a_(I){circumflex over(m)}₂×({circumflex over (m)}₂×{circumflex over (m)}₁), where {circumflexover (m)}₂ is a unit vector in the direction of the magnetic moment ofthe free magnetic layer FM2 and {circumflex over (m)}₁ is a unit vectorin the direction of the magnetic moment of the pinned magnetic layerFM1. The prefactor, a_(I), depends on the current I, thespin-polarization P of the current I, and the cosine of the anglebetween the free and pinned magnetic layers, cos(θ), such that a_(I)=

Ig(P, cos(θ))/(mMV).

is the reduced Planck's constant, g is a function of thespin-polarization P and cos(θ), M is the magnetization density of thefree layer, m is the mass of the electron, and V is the volume of thefree layer (see, J. Slonczewski, Journal of Magnetism and MagneticMaterials 159, L1 (1996)). Thus, a large torque {right arrow over(τ)}_(S) is produced when the magnetic moments of the pinned magneticlayer FM1 and the free magnetic layer FM2 are perpendicular.

This torque {right arrow over (τ)}_(S), which acts on the magneticmoment of the free magnetic layer FM2, causes the magnetization of thefree magnetic layer FM2 to rotate out of the plane of the layer. Sincethe thickness of the free magnetic layer FM2 is less than the width andlength dimensions of the free magnetic layer FM2, the rotation of themagnetization vector {right arrow over (m)}₂ of the free magnetic layerFM2 out of the plane of the layer generates a large magnetic field, a‘demagnetizing’ field, which is perpendicular to the plane of the layer.

This demagnetizing field forces the magnetization vector {right arrowover (m)}₂ of the free magnetic layer FM2 to precess, i.e., to move suchthat the magnetization direction rotates about the magnetic field axis.The demagnetizing field also determines the rate of precession. A largedemagnetizing field results in an extremely high precession rate, whichis an optimal condition for fast magnetic switching.

Thus, in an optimal configuration of the magnetic memory device for fastmagnetic switching, the magnetic moment of the pinned magnetic layer FM1is perpendicular to the plane of the free magnetic layer FM2, and themagnetic moment of the free magnetic layer FM2 is perpendicular to theaxis of the pillar of thin layers and lies in the plane of the freemagnetic layer FM2.

FIG. 2E shows the free magnetic layer FM2 after the magnetic switchingprocess is completed. As shown in FIGS. 2A and 2E, the magneticswitching process causes the magnetization vector {right arrow over(m)}₂ of the free magnetic layer FM2 to switch by reversing direction byrotating 180°.

FIGS. 3A and 3B show two different forms of current input that may beapplied to the magnetic device. The current input shown in FIG. 3A iscomprised of two current pulses of short duration, a first positivecurrent pulse followed by a second negative current pulse. This form ofcurrent input results in writing a ‘1’ or a ‘0’. Alternatively, thefirst current pulse can be negative and the second current pulse can bepositive, as long as the two current pulses are of opposite polarity. Inboth cases, the state of the magnetic bit will be changed from ‘1’ to‘0’ or ‘0’ to ‘1’ (i.e., the final state will be the complement of theinitial state of the bit). The current input shown in FIG. 3A is used inthe method of magnetic switching described above and shown in FIGS.2A–2E. Using a current input formed of two current pulses results in afaster magnetic switching process.

The first current pulse starts the precession of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2. After thecompletion of the first current pulse, the second current pulse isapplied to stop the precession at a desired state.

The second current pulse is not essential to the operation of thedevice, but it enables higher speed switching. For example, the currentinput shown in FIG. 3B is comprised of a single positive current pulse.Alternatively, a single negative current pulse may also be applied tothe magnetic device. Simulations show that many different types ofcurrent pulses switch FM2. Therefore device operation is certainly notlimited to the current pulses shown in FIG. 3.

Structure of a Memory Cell

The magnetic device described above can be incorporated into a memorycell for inclusion into arrays of memory cells to make up a magneticmemory. According to one embodiment as shown in FIG. 4, the magneticdevice of the present invention, when implemented as a memory cell, is amultilayered, pillar-shaped device having a pinned magnetic layer FM1with a fixed magnetization direction, a free magnetic layer FM2 with afree magnetization direction, and a read-out magnetic layer FM3 with afixed magnetization direction. {right arrow over (m)}₁ is themagnetization vector of the pinned magnetic layer FM1, {right arrow over(m)}₂ is the magnetization vector of the free magnetic layer FM2, and{right arrow over (m)}₃ is the magnetization vector of the read-outmagnetic layer FM3.

The pinned magnetic layer FM1 and the free magnetic layer FM2 areseparated by a first non-magnetic layer N1 that spatially separates thetwo layers FM1 and FM2 such that their mutual magnetic interaction isminimized. The free magnetic layer FM2 and the read-out magnetic layerFM3 are separated by a second non-magnetic layer N2 that spatiallyseparates the two layers FM2 FM3 such that their mutual magneticinteraction is minimized. The pillar-shaped magnetic device is typicallysized in nanometers, e.g., it may be less than approximately 200 nm.

An electric current source is connected to the pinned magnetic layer FM1and the read-out magnetic layer FM3 so that an electric current I cantraverse the pillar device. A voltmeter is connected to the pinnedmagnetic layer FM1 and the read-out magnetic layer FM3 so that theresistance of the magnetic device can be measured to thereby read thelogical contents of the memory cell.

Method for Writing Information

The magnetic switching process is used when information is written intoa memory cell. To store a logical bit of information in a memory cell,the magnetization direction of the magnetization vector inside thememory cell is set in one of two possible orientations to code thelogical values of ‘0’ and ‘1’. This magnetic device, when implemented asa memory cell, uses the method of magnetic switching describedpreviously in order to store bits of information. Current pulses areapplied to change the logical value in the magnetic device. The magneticmemory device described above and shown in FIG. 4 stores one bit ofinformation since the free magnetic layer FM2 has a single magnetizationvector {right arrow over (m)}₂ with two stable magnetic states.

An electric current I is applied to the pillar-shaped magnetic memorydevice so that the current I flows through the various layers of themagnetic memory device, from the pinned magnetic layer FM1 to theread-out magnetic layer FM3. The applied current I results in a transferof angular momentum from the pinned magnetic layer FM1 to the freemagnetic layer FM2.

FIGS. 5A–5E show steps in the method of writing information using themagnetic memory device shown in FIG. 4 and for convenience, FIGS. 5A–5Eonly show the free magnetic layer FM2 and the magnetization vector{right arrow over (m)}₂ of the free magnetic layer FM2. FIG. 5A showsthe initial state of the free magnetic layer FM2 before the current I isapplied.

As shown in FIGS. 5B–5D, applying a current I, which can be of a form asshown in FIGS. 3A and 3B, results in the transfer of angular momentumfrom the pinned magnetic layer FM1 to the free magnetic layer FM2. FIGS.2A–2E and 5A–5E show the change in the orientation of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2 as aresult of applying the current to the magnetic device.

FIG. 6A shows a form of the current input that is applied to themagnetic memory device shown in FIG. 4. The current input of FIG. 6Aincludes two current pulses of short duration, a first positive currentpulse followed by a second negative current pulse, which results inwriting a ‘1’ or a ‘0’. Alternatively, the first current pulse can benegative and the second current pulse can be positive, as long as thetwo current pulses are of opposite polarity. In both cases, the state ofthe magnetic bit will be changed from ‘1’ to ‘0’ or ‘0’ to ‘1’ (i.e.,the final state will be the complement of the initial state of the bit).

The first current pulse starts the precession of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2. After thecompletion of the first current pulse, the second current pulse isapplied to stop the precession at a desired state. For this embodimentof the magnetic memory device of the present invention, the precessionis stopped when 180° rotation of the magnetization vector {right arrowover (m)}₂ of the free magnetic layer FM2 is achieved.

FIG. 6B shows an example of the corresponding resistance of the deviceas measured by the voltmeter connected to the magnetic memory deviceshown in FIG. 4 with a small current applied, i.e., a current intensitymuch less than that used in the current pulses. The resistance increasesafter the current pulses of FIG. 6A are applied to the device. At theinitial state shown in FIG. 5A (before the first positive currentpulse), the resistance is at a constant low value. At the final stateshown in FIG. 5E, the resistance is at a constant high value.

Thus, the states shown in FIGS. 5A and 5E correspond to a logical valueof “0” in the initial state and a logical value of “1” in the finalstate, respectively. The magnetization vector {right arrow over (m)}₂ ofthe free magnetic layer FM2 in the final state shown in FIG. 5E is inthe opposite direction than the magnetization vector {right arrow over(m)}₂ of the free magnetic layer FM2 in the initial state shown in FIG.5A.

The necessary amplitude of the current pulses can be estimated bynumerical modeling using the equations of micromagnetics, theLandau-Lifzshitz Gilbert equations including the spin-transfer torquediscussed earlier (see, for example, B. Oezyilmaz et al., Phys. Rev.Lett. 91, 067203 (2003)). For a free layer comprised of Co with amagnetization density of M=1400 emu/cm³, a Gilbert damping parameter αof 0.01, a spin-polarization of the current P of 0.4, and an in-planeuniaxial anisotropy field of 1000 kOe. (In this case, the in-planeuniaxial anisotropy constant K is K=7×10⁵ erg/cm³.) For the purposes ofthis estimation, the Co free layer is 3 nm thick and has lateraldimensions of 60 nm by 60 nm. We find that a current pulse of amplitudeof 5 mA is more than sufficient to switch the layer. The currentnecessary to switch the device is reduced by decreasing the size of theCo free layer; increasing the spin-polarization of the current, forexample, by using a pinned layer with a higher degree ofspin-polarization; and decreasing the in-plane anisotropy or decreasingthe Gilbert damping. For this current amplitude, a 35 psec pulse issufficient to switch the device.

With a device resistance of 5 Ohms, the energy dissipation is 5×10⁻¹⁵ J.This energy dissipation value can be compared to the energy needed toswitch a magnetic device with a spin-polarized current when the pinnedlayer and the free layer magnetizations are initially aligned along thesame axis. Recent experiments show that this requires a current ofapproximately 10 mA applied for approximately 10 ns in a device with aresistance of 5 Ohms (see, R. Koch et al., preprint to be published inPhys. Rev. Lett.). The energy dissipated is thus 5×10⁻¹² J. Thus, incomparison, the power requirement for our device is quite small.Further, because the pulse is on only very briefly, in spite of thelarge current densities, 1 A/μm², no electromigration is expected.Further, we have operated such devices at current densities 5 timesgreater than this value for extended periods (approximately 1 minute)with no device damage (see, B. Oezyilmaz et al., Phys. Rev. Lett. 91,067203 (2003)).

Method for Reading Information

The read-out magnetic layer FM3 is required in the simplestimplementation of the magnetic memory device. The read-out magneticlayer FM3 has a magnetization vector {right arrow over (m)}₃ with afixed magnetization direction. The magnetization vector {right arrowover (m)}₃ of the read-out magnetic layer FM3 can be fixed in a numberof ways. For example, the read-out magnetic layer FM3 can be formedthicker or of a higher anisotropic magnetic material or can be placedadjacent to an antiferromagnetic layer to use the phenomena of exchangebiasing. In the phenomena of exchange biasing, the coupling between theantiferromagnetic layer and the ferromagnetic layer and the largemagnetic anisotropy of the antiferromagnetic layer results in ahardening of the ferromagnetic layer so that larger magnetic fields andcurrents are required to change its magnetization direction.

The resistance of the magnetic memory device is very sensitive to therelative orientation of the magnetization vector {right arrow over (m)}₂of the free magnetic layer FM2 and the magnetization vector {right arrowover (m)}₃ of read-out magnetic layer FM3. The resistance of themagnetic memory device is highest when the magnetization vectors {rightarrow over (m)}₂ and {right arrow over (m)}₃ of the free magnetic layerFM2 and the read-out layer FM3, respectively, are in anti-parallelalignment. The resistance of the magnetic device is lowest when themagnetization vectors {right arrow over (m)}₂ and {right arrow over(m)}₃ of the layers FM2 and FM3, respectively, are in parallelalignment. Thus, a simple resistance measurement can determine theorientation of the magnetization vector {right arrow over (m)}₂ of thefree magnetic layer FM2.

The fixed orientation of the magnetization vector {right arrow over(m)}₃ of the read-out magnetic layer FM3 is set so that it is either inparallel or anti-parallel alignment with the magnetization vector {rightarrow over (m)}₂ of the free magnetic layer FM2, depending on theorientation of the magnetization vector {right arrow over (m)}₂ of thefree magnetic layer FM2. Since the orientation of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2 switchesso that it can be rotated 180°, the magnetization vectors {right arrowover (m)}₂ and {right arrow over (m)}₃ of the free magnetic layer FM2and the read-out layer FM3, respectively, must be in eitheranti-parallel or parallel alignment.

Storage of Multiple Bits of Information

The magnetic memory device described above and shown in FIG. 4 has twostable magnetic states and is able to store one bit of information.According to another embodiment of the present invention, a magneticmemory device can be constructed to store multiple bits of information.FIG. 7 shows an example of a free magnetic layer FM2 with four stablemagnetic states. A magnetic memory device comprising a free magneticlayer FM2 with four stable magnetic states is able to store two bits ofinformation. In this embodiment, current pulses are applied to switchthe magnetization between directions that differ by 90° instead of 180°.This can be accomplished by current pulses of a different form. Forexample, the current pulses can be smaller in amplitude and/or shorterin duration. The readout layer (FM3) is then aligned such that each ofthe four magnetization states has a different resistance. This requiresthat the readout layer magnetization not have an in-plane component thatpoints parallel to any of the four states nor at 45° to these states.

EXAMPLE

The operation of the magnetic device was simulated usingLandau-Lifzshitz Gilbert equations including a spin-transfer torque.

FIG. 8 shows the amplitude of the current input applied to the magneticmemory device starting at an initial time t=0 and ending at t=30picoseconds. This current input comprises two current pulses similar tothe current input shown in FIGS. 3A and 6A.

A 16-picosecond positive current pulse is applied to the magnetic memorydevice to start the precession of the magnetization vector {right arrowover (m)}₂ of the free magnetic layer FM2. After this 16-picosecondcurrent pulse, a 14-picosecond negative current pulse is applied to themagnetic memory device to stop the precession of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2 to achievea desired state of the magnetization vector {right arrow over (m)}₂. Formagnetic memory devices, the precession is stopped after achieving a180° rotation of the magnetization vector {right arrow over (m)}₂ of thefree magnetic layer FM2.

FIG. 9 shows the magnetization components m_(X) and m_(Y) of themagnetization vector {right arrow over (m)}₂ of the free magnetic layerFM2 in the x- and y-directions shown in FIGS. 2B and 5B. Themagnetization components m_(X) and m_(Y) are measured during and afterthe application of the current input shown in FIG. 8. FIG. 9 shows thatthe magnetization vector {right arrow over (m)}₂ of the free magneticlayer FM2 reverses 1800 from the initial state, which corresponds toFIG. 5A, to the final state, which corresponds to FIG. 5E. Themagnetization components (m_(X), m_(Y)) are able to switch between(−1,0) to (1,0) as shown by the present invention.

ADVANTAGES

The high speed, low power magnetic device of the present invention usesenergy only for read and write operations or logic operations. When notenergized, the information is stored without significant loss. Thus, themagnetic device of the present invention, when implemented as a memorycell, can be used as a non-volatile memory.

The non-volatile memory provided by the magnetic device of the presentinvention is suitable for many applications, such as in computers andportable electronic devices. In particular, the high speed, low powermagnetic device of the present invention provides several advantages.The performance of the high speed, low power magnetic device of thepresent invention compares favorably with flash memory and other typesof non-volatile random access memory (RAM), such as conventionalmagnetic RAM (MRAM) and ferroelectric RAM (FRAM).

The current-induced torques act only on the magnetic device that isenergized, i.e., to which a current is applied. Therefore, when multiplemagnetic devices are arranged in an array, such as in magnetic memory,the current-induced spin transfer does not produce parasiticinteractions (“cross-talk”) between the neighboring elements in thearray, unlike in conventional magnetic memories in which magneticswitching is accomplished by using magnetic fields produced by smallcurrent-carrying wires near the magnetic elements.

The method of magnetic switching by current induced torque provided bythe present invention is faster than current conventional methods thatuse magnetic fields to switch the magnetization direction of layers.Read-out and write operations of the present invention can be completedin sub-nanosecond time scales. Conventional magnetic hard drives arevery slow compared to the magnetic memory of the present invention sincethe conventional hard drives have data access times of the order ofmilliseconds.

The method of magnetic switching by current induced torque provided bythe present invention requires low power. This is especiallyadvantageous for use in portable electronic devices.

The method of magnetic switching by current induced torque provided bythe present invention is ideal for sub-micron scale devices since thelateral dimension of the magnetic device of the present invention may beless than approximately 200 nm. Therefore, the present invention isscaled to allow the fabrication of ultra-high density memory cells sothat a vast amount of information can be stored in the magnetic memoryprovided by the present invention.

The basic architecture of the high speed, low power magnetic device ofthe present invention is straightforward, and read-out and writeoperations are reliable and are less sensitive to changes intemperature. Unlike conventional magnetic memory devices, the presentinvention does not rely on stochastic (random) processes or fluctuatingfields to initiate switching events.

According to one embodiment of the present invention, multiple bits ofinformation can be stored on each device so that even more informationcan be stored in the magnetic memory.

The method of magnetic switching by current induced torque provided bythe present invention can be used for logic operations, as well as formagnetic memory devices. Since there is a threshold, which is dependenton the shape, amplitude, and period of the current pulse, for thecurrent pulse to produce a change in magnetization, current input can becombined to produce a logic function, such as an AND gate. For example,two current pulses can be combined to produce a current pulse thattraverses the device which is the sum of the two current pulses. Thepulse characteristics (shape, amplitude, and period) can be chosen suchthat each pulse individually does not switch the device, yet thecombined pulse does switch the device. Thus, this is an AND operation. ANOT operation requires simply switching the state of the device. A NOTand an AND operation can be combined to produce a NAND function, whichis a universal digital logic gate (i.e., all digital logic functions canbe constructed from NAND gates.)

There are several possible geometries and layer configurations that areprovided by the present invention. For example, an embodiment of themagnetic device of the present invention may be configured so that nonet current passes through the free magnetic layer FM2 during writeoperations. This is illustrated in FIG. 10 which shows an embodiment ofthe present invention including current source A, current source B, andlayer I2, which is a thin insulating layer made of Al₂O₃, for example.In this device, layer I2 is 0.5 to 3 nm thick and is thin enough so thatelectrons can traverse the layer by quantum mechanical tunneling.

In the device shown in FIG. 10, current pulses are applied with currentsource A to change the magnetization direction of the free magneticlayer FM2. Using current source A, current flows from FM1 to thenon-magnetic layer N1 and electron spin angular momentum is transferredto the free magnetic layer FM2 by reflection of electrons at theinterface between the non-magnetic layer N1 and the free magnetic layerFM2. The device readout is performed using current source B. The voltageis measured when a small current from B passes between the free magneticlayer FM2 and the readout layer FM3. This voltage will depend on therelative magnetization directions of the layers FM2 and FM3 so that themagnetization direction of the free magnetic layer FM2 can be determinedto read-out the device. This device has the advantage that the readoutsignal is large since the tunnel junction resistance can be large (1 Ohmto 100 kOhm). Readout signals can be in the range from 10 mV to 1 V.

While there has been described what are at present considered to beembodiments of the present invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaims cover all such modifications as fall within the true spirit andscope of the invention.

1. A method of magnetic switching using current induced spin-momentumtransfer, said method comprising the steps of: applying an electriccurrent to a magnetic device comprising a pinned magnetic layer having apinned magnetic vector and a free magnetic layer having a magnetizationvector with at least a first and second stable magnetic state vector;transferring the spin-momentum of the pinned magnetic layer to the freemagnetic layer while the electric current is applied; and stopping saidelectric current application when the magnetization vector of the freemagnetic layer changes from one of said magnetic state vectors to theother of said magnetic state vectors.
 2. The method of magneticswitching according to claim 1, wherein the magnetic device furthercomprises a read-out magnetic layer having a fixed magnetization vector.3. The method of magnetic switching according to claim 2 furthercomprising the step of: utilizing the fixed magnetization vector of theread-out magnetic layer to determine a state of the free magnetic layer.4. The method of magnetic switching according to claim 1, wherein saidelectric current comprises two current pulses wherein one of said twocurrent pulses includes a negative current pulse and the other of saidtwo current pulses includes a positive current pulse.
 5. The method ofmagnetic switching according to claim 2, wherein the first stablemagnetic state vector is parallel to the magnetization vector of theread-out magnetic layer, and the second stable magnetic state vector isanti-parallel to the magnetization vector of the read-out magneticlayer.
 6. The method of magnetic switching according to claim 2, whereinthe free magnetic layer further includes third and fourth stablemagnetic state vectors, said first, second, third and fourth stablemagnetic state vectors being coplanar with each other and each orientedin a respective direction such that the directions are separated byapproximately 90°, and the magnetization vector of the read-out layerhaving an in-plane component that is neither parallel nor separated by45° with respect to each of the stable magnetic state vectors of thefree magnetic layer, and the stopping step is performed when themagnetization vector of the free magnetic layer has rotated to one ofthe at least first, second, third, and fourth stable magnetic statevectors.
 7. The method of magnetic switching according to claim 2,further comprising the step of applying the current through the pinnedmagnetic layer, the free magnetic layer, and the read-out magneticlayer.
 8. A method of writing and reading data in a magnetic memorydevice comprising the steps of: applying an electric current to amagnetic device comprising a pinned magnetic layer having a pinnedmagnetic vector and a free magnetic layer having a magnetization vectorwith at least first and second stable magnetic state vectors, and aread-out magnetic layer having a fixed magnetization vector, wherein theelectric current comprises two current pulses wherein one of the twocurrent pulses includes a negative current pulse and the other of thetwo current pulses includes a positive current pulse; transferring thespin-momentum of the pinned magnetic layer to the free magnetic layerwhile the electric current is applied; and stopping the electric currentwhen the magnetization vector of the free magnetic layer has rotated toone of the at least first and second stable magnetic state vectors whilethe electric current is applied; and measuring a resistance through atleast the read-out magnetic layer and the free magnetic layer.
 9. Themethod of writing and reading data in a magnetic memory device accordingto claim 8, further comprising the step of: utilizing the fixedmagnetization vector of the read-out magnetic layer to determine a stateof the free magnetic layer of the magnetic device.
 10. The method ofwriting and reading data in a magnetic memory device according to claim8, further comprising the step of: utilizing the pinned magnetizationvector of the pinned magnetic layer to determine a state of the freemagnetic layer of the magnetic device.
 11. The method of writing andreading data in a magnetic memory device according to claim 8, whereinthe current applying step occurs in a sub-nanosecond period of time. 12.The method of writing and reading data in a magnetic memory deviceaccording to claim 8, wherein the first stable magnetic state isparallel to the magnetization vector of the read-out magnetic layer, andthe second stable magnetic state is anti-parallel to the magnetizationvector of the read-out magnetic layer.
 13. The method of writing andreading data in a magnetic memory device according to claim 8, whereinthe free magnetic layer further includes third and fourth stablemagnetic state vectors, said first, second, third, and fourth stablemagnetic state vectors being co-planar with each other and each orientedin a respective direction such that the directions are separated byapproximately 90°, and the magnetization vector of the read-out layerhaving an in-plane component that is neither parallel nor separated by45° with respect to each of the stable magnetic state vectors of thefree magnetic layer, and the stopping step is performed when themagnetization vector of the free magnetic layer has rotated to one ofthe at least first, second, third, and fourth stable magnetic statevectors.
 14. The method of writing and reading data in a magnetic memorydevice according to claim 8, further comprising the step of applyingcurrent through the pinned magnetic layer, the free magnetic layer, andthe read-out magnetic layer.
 15. The method of writing and reading datain a magnetic memory device according to claim 8, further comprising thestep of measuring the resistance across the magnetic device.
 16. Amagnetic device comprising: a pinned magnetic layer with a magnetizationvector having a fixed magnetization direction; a free magnetic layerwith at least one magnetization vector having a changeable magnetizationdirection; and a first non-magnetic layer spatially separating said freemagnetic layer and said pinned magnetic layer.
 17. The magnetic deviceaccording to claim 16, wherein one of said magnetization directions ofsaid pinned magnetic layer and said free magnetic layer lie along anaxis which is different than at least one of the axes along which theother magnetization directions lie.
 18. The magnetic device according toclaim 16 further comprising: a read-out magnetic layer with amagnetization vector having a fixed magnetization direction; and asecond non-magnetic layer spatially separating said free magnetic layerand said read-out magnetic layer.
 19. The magnetic device according toclaim 18, wherein one of said magnetization directions of said pinnedmagnetic layer, said free magnetic layer, and said read-out magneticlayer lies along an axis which is different than at least one of theaxes along which the other magnetization directions lie.
 20. Themagnetic device according to claim 16, wherein: said fixed magnetizationdirection of said pinned magnetic layer is perpendicular to a plane ofsaid free magnetic layer; and said changeable magnetization direction ofsaid free magnetic layer is perpendicular to an axis extendinglongitudinally through said magnetic device.
 21. The magnetic deviceaccording to claim 18, wherein said changeable magnetization directionof said free magnetic layer and said fixed magnetization direction ofsaid read-out layer switch between being in anti-parallel alignment andparallel alignment.
 22. The magnetic device according to claim 16,wherein the mutual magnetic interaction between said free magnetic layerand said pinned magnetic layer is minimized.
 23. The magnetic deviceaccording to claim 18, wherein said magnetization direction of saidmagnetization vector of said free magnetic layer represents a bit ofinformation.
 24. The magnetic device according to claim 16, wherein:said magnetic device is substantially pillar-shaped and having acharacteristic dimension less than 200 nm; and said pinned magneticlayer, said first non-magnetic layer, and said free magnetic layer areeach approximately 1 nm to 50 nm thick.
 25. The magnetic deviceaccording to claim 18, wherein: said magnetic device is substantiallypillar-shaped and having a characteristic dimension less than 200 nm;and said pinned magnetic layer, said first non-magnetic layer, said freemagnetic layer, said second non-magnetic layer, and said read-outmagnetic layer are each approximately 1 nm to 50 urn thick.
 26. Themagnetic device according to claim 16, wherein said pinned magneticlayer and said free magnetic layer are comprised of a member of thegroup consisting of Co, Ni, Fe, an alloy of Co and Ni, an alloy of Coand Fe, an alloy of Ni and Fe, an alloy of Co, Ni, and Fe, and permalloyNi_(1-x)Fe_(x).
 27. The magnetic device according to claim 18, whereinsaid pinned magnetic layer, said free magnetic layer, and said read-outmagnetic layer are comprised of a member of the group consisting of Co,Ni, Fe, an alloy of Co and Ni, an alloy of Co and Fe, an alloy of Ni andFe, an alloy of Co, Ni, and Fe, and permalloy Ni_(1-x)Fe_(x).
 28. Themagnetic device according to claim 16, wherein said pinned magneticlayer and said free magnetic layer are comprised of a non-magnetic metaland a member of the group consisting of an alloy of Co and Ni, an alloyof Co and Fe, an alloy of Ni and Fe, an alloy of Co, Ni, and Fe, suchthat said non-magnetic metal and said member are ferromagneticallyordered at room temperature.
 29. The magnetic device according to claim18, wherein said pinned magnetic layer, said free magnetic layer, andsaid read-out magnetic layer are comprised of a non-magnetic metal and amember of the group consisting of an alloy of Co and Ni, an alloy of Coand Fe, an alloy of Ni and Fe, an alloy of Co, Ni, and Fe, such thatsaid non-magnetic metal and said member are ferromagnetically ordered atroom temperature.
 30. The magnetic device according to claim 28, whereinsaid non-magnetic metal is a member of the group consisting of Cu, Pd,Pt, and NiMnSb.
 31. The magnetic device according to claim 29, whereinsaid non-magnetic metal is a member of the group consisting of Cu, Pd,Pt, and NiMnSb.
 32. The magnetic device according to claim 16, whereinsaid pinned magnetic layer and said free magnetic layer comprise eitherCrO₂ or Fe₃O₄.
 33. The magnetic device according to claim 16, whereinsaid non-magnetic layer is comprised of at least one member of the groupconsisting of Cu, Cr, Au, Ag, and Al.
 34. The magnetic device accordingto claim 18, wherein said non-magnetic layers are comprised of at leastone member of the group consisting of Cu, Cr, Au, Ag, and Al.
 35. Themagnetic device according to claim 18, wherein the mutual magneticinteraction between said free magnetic layer and said read-out magneticlayer is minimized.
 36. A memory system including a memory cellcomprising: a pinned magnetic layer with a magnetization vector having afixed magnetization direction; a free magnetic layer with at least onemagnetization vector having a changeable magnetization direction; afirst non-magnetic layer spatially separating said free magnetic layerand said pinned magnetic layer; a read-out magnetic layer with amagnetization vector having a fixed magnetization direction; and asecond non-magnetic layer spatially separating said free magnetic layerand said read-out magnetic layer.
 37. The memory system according toclaim 36, wherein one of said magnetization directions of said pinnedmagnetic layer, said free magnetic layer, and said read-out magneticlayer lies along an axis which is different than at least one of theaxes along which said other magnetization directions lie.
 38. The memorysystem according to claim 36, further comprising a means for measuringthe resistance between said pinned magnetic layer and said read-outmagnetic layer.
 39. The memory system according to claim 38, whereinsaid resistance measuring means comprises a voltmeter connected to saidpinned magnetic layer and said read-out magnetic layer.
 40. The memorysystem according to claim 36, further comprising an electric currentsource connected to said pinned magnetic layer and said read out layerso that an electric current can traverse said memory cell, wherein saidelectric current comprises at least one current pulse.
 41. The memorysystem according to claim 40, wherein said electric current comprisestwo current pulses, wherein one of said two current pulses is a negativecurrent pulse and the other of said two current pulses is a positivecurrent pulse.
 42. The memory system according to claim 40, wherein saidelectric current is applied in a sub-nanosecond period of time.
 43. Thememory system according to claim 36, wherein the mutual magneticinteraction between said free magnetic layer and said read-out magneticlayer is minimized.
 44. A method of performing a logical operationcomprising the steps of: applying first and second electric currentinput signals to a magnetic device comprising a pinned magnetic layerhaving a pinned magnetic vector, a free magnetic layer having amagnetization vector with at least a first and second stable magneticstate vector, and a read-out magnetic layer having a fixed magnetizationvector, wherein said first and second stable magnetic state vectors arenot parallel to the pinned magnetic vector; and transferring thespin-momentum of the pinned magnetic layer to the free magnetic layerwhile the first and second electric current input signals are applied sothat the magnetization vector of the free magnetic layer changes fromone of said magnetic state vectors to the other of said magnetic statevectors if a sum of the first and second current input signals exceeds apredetermined level; else, maintaining the magnetization vector of thefree magnetic layer if the sum of the first and second current inputsignals does not exceed the predetermined level.
 45. The method ofperforming a logical operation according to claim 44, further comprisingthe step of: switching the magnetization vector of the free magneticlayer from one of said magnetic state vectors to the other of saidmagnetic state vectors.
 46. The memory system according to claim 36,wherein the mutual magnetic interaction between said free magnetic layerand said pinned magnetic layer is minimized.